The Iliad is the first great book, and the first great book about the suffering and loss of war. We love to tell stories about war. Tony Blair wove his own when giving evidence at the Chilcot inquiry yesterday: the latest, unpoetic attempt to make sense of an east-west clash of powers. He might note that "spin " goes back to The Iliad: the first-century writer Dio Chrysostom argued that Homer, for reasons of his own, suppressed the truth about the Trojan war – in reality, the Greeks lost. "Men learn with difficulty . . . but they are deceived only too readily," he wrote.

Why is the first book a book about war? Perhaps because war is inextricably bound up with humanity's urge to tell stories. Civilisation – with its settlements, its boundary lines, its hierarchies – breeds conflict and narrative alike. In The Iliad, two characters have the narrative urge, and something approaching a synoptic view of the scenes surging around them. Achilles sings stories of heroes' deeds in battle, and Helen embroiders scenes of fighting on an elaborate textile.

Many wishing to make sense of wars in their own time have reached for The Iliad. Alexander the Great, perhaps the most flamboyantly successful soldier in history, slept beside a copy annotated by his tutor, Aristotle. "He esteemed it a perfect portable treasure of all military virtue and knowledge," according to Plutarch's biography. Simone Weil's essay, "L'Iliade ou le poème de la force", published in 1940, holds that "the true hero, the true ­subject at the centre of The Iliad is force", which she defines as "that X that turns anybody who is subjected to it into a thing".

Her contemporary Rachel Bespaloff, a Geneva-raised philosopher who wound up in the United States, also turned to Homer's poem as a "method of facing" the second world war. For her, it tells a profound, human story – "Suffering and loss have stripped Hector bare," her essay "On The Iliad" begins.

We are still turning to The Iliad, amid our own wars: the Australian writer David Malouf's recent novel, Ransom (Chatto & Windus), is about the encounter between Priam and Achilles in The Iliad's final book, while Caroline Alexander's new study of the poem, The War that Killed Achilles (Faber), sees it as a meditation on the catastrophic effects of conflict. While she does not indulge in crass equivalences, it is hard not to be alerted by her reading to the devastation caused by the conflicts in Afghanistan and Iraq.

Today's students at West Point, the elite US military academy where one may minor in "terrorism studies", study The Iliad as part of their literature course. In her 2007 book Soldier's Heart, Elizabeth Samet, literature professor at the institution, recalls a visit by the late translator-poet Robert Fagles, who recited, in Greek, the first lines of the epic. The 1,000 plebes in his audience must now be in command positions in Iraq and Afghanistan. The military language of the conflicts even brings with it distant echoes of Homer: Operation Achilles was a Nato offensive in 2007 aimed at clearing Helmand province of the Taliban.

The Trojan war – a more or less mythical event – was a 10-year siege of the city of Troy by a coalition of Greeks, its purpose to restore Helen to her Spartan husband, Menelaus. The Iliad charts not the famous causes of the conflict (the Trojan prince Paris's abduction of Helen) nor its ­spectacularly bloody end (the Greeks' ruse of the wooden horse and the brutal sacking of the city). Instead, the subject of the poem is menis, fury – specifically, the wrath of the Greeks' best warrior, Achilles.

That wrath is provoked by his ­commander-in-chief Agamemnon's misguided decision to seize Briseis, Achilles's captive woman, as compensation for his own bit of living loot, Chriseis, whom he has been obliged to restore to her Trojan father. Achilles, his pride and honour outraged, withdraws from the fighting and persuades his mother, the goddess Thetis, to ask Zeus to turn the tide of war against the Greeks, knowing that they will suffer appalling losses. He stubbornly resists all appeals to return to battle, but ­eventually agrees to send his beloved comrade, Patroclus, into the fray.

When Patroclus is killed by the Trojans' best fighter, Hector, Achilles whirls into a frenzy of redoubled, re­directed rage. He joins the fighting, and begins a lengthy and pitiless slaughtering spree. Finally, he kills Hector in single combat and attaches the corpse to his chariot, dragging it triumphantly around the walls of the city. (In 2004, the bodies of American contractors were attached to the backs of cars and dragged through the streets of Fallujah.) At the end of the poem Hector's frail and eldery father, Priam, enters the Greeks' camp and persuades Achilles to restore to him his son's body.

Not all soldiers have seen the point. TE Lawrence esteemed Homer sufficiently to translate him (rather unsatisfactorily), but he was scornful of the poet's knowledge of military affairs. Homer, he thought, must have been "very bookish" and "a house-bred man". In her book Samet records one of her students, declaring that ­"Alexander was a fool to carry this poem around with him." He had found ­nothing to emulate in either Agamemnon or Achilles – until he read through to book 11 of the poem, when he "got" it. This is the section known as Agamemnon's aristeia – his day of glory in the field. Perhaps what appealed to the student was the scene in which the commander arms for ­battle, around 30 lines of minutely ­described military hardware down to the bronze-tipped spears that flash in the sunlight's glare: lovingly summoned-up boys' toys. Or perhaps, ­after all, it was the ­account of Agamemnon's brutal military prowess that transfixed him, the commander knocking the life out of every young Trojan he encounters, deaf to their cries for mercy:

"And he pitched Pisander off the chariot on to earth

and plunged a spear in his chest – the man crashed on his back as

Hippolochus leapt away, but him he killed on the ground,

slashing off his arms with a sword, lopping off his head

and he sent him rolling through the carnage like a log."

(Here, as throughout, the translation is Fagles's for Penguin Classics.)

The onward rush of these almost joyful descriptions of slaughter in The Iliad might cause some modern readers to question the values of the poem, or at least to measure out the long distance between us and the society from which it sprang. Homer was no peacenik. "Homer and Tolstoy have in common a virile love of war and a virile horror of it," Bespaloff wrote in "On The Iliad". It is futile to look to Homer for a condemnation of war: "People make war, they put up with it, they curse it, they even praise it in songs and verses, but it is not to be judged any more than destiny is."

But it's easy to see why Lawrence struggled to admire The Iliad's descriptions of battle. Though they are never lacking in drama, they are frequently implausible, even to a civilian eye, not least in the way that soldiers die – ­impossibly cleanly and instantaneously. Rare are the cases in which the combatants are tended to by literature's first field surgeons, Machaon and Podalirius, or on one occasion by Patroclus himself, who turns medic to help his comrade Eurypylus. The agony of death-throes, the cries of pain from soldiers too wounded to move, are absent from the poem. Compare this account, by John Charles Austin, from John Carey's Faber Book of Reportage, describing the evacuation of the British Expeditionary Force from Dunkirk in June 1940: "A horrible stench of blood and mutilated flesh pervaded the place . . . We set our faces in the direction of the sea, quickening our pace to pass through the belt of this nauseating miasma as soon as possible. 'Water . . . Water . . .' groaned a voice from the ground just in front of us. It was a wounded infantryman. He had been hit so badly that there was no hope for him."

Nor do the heroes of The Iliad suffer the long-term consequences of injury – a fact for which the disparity between ancient and modern medical practice cannot alone account. Odysseus famously has a scar in The Odyssey – it is the means by which his childhood nurse, Eurycleia, sees through his ­disguise as she bathes him on his return to Ithaca – but this he acquired in a boar hunt.

Yet The Iliad still has much to say about war, even as it is fought today. It tells us that war is both the bringer of renown to its young fighters and the destroyer of their lives. It tells us about post-conflict destruction and chaos; about war as the great reverser of fortunes. It tells us about the age-old dilemmas of fighters compelled to serve under incompetent superiors. It tells us about war as an attempt to protect and preserve a treasured way of life. It tells us, too, about the profound gulf between civilian existence and life on the front line; about atrocities and indiscriminate slaughter; about war's peculiar mercilessness to women and children; about friendships and sympathies across the battle lines. It tells us of the love between soldiers who fight together. Most of all, it tells us about the frightful losses of war: of a soldier losing his closest companion, of a ­father losing his son.

At the centre of the poem's most urgent observations on the nature of war is its hero, Achilles, an extreme character in all senses – The Iliad's most bloodthirsty warrior, the quickest to anger, but at times the most tender. He is tinged with the supernatural: his mother is a goddess; his armour is forged by the god Hephaestus; even his chariot-team consists of immortal horses, the gift of Zeus. He sees the war with an enhanced perspective; as Alexander points out, he is clear-eyed about the utter pointlessness of the conflict. During his outburst to Agamemnon in book one, Achilles says:

The Trojans never did me damage, not in the least,

they never stole my cattle or my horses, never

in Phthia where the rich soil breeds strong men

did they lay water my crops. How could they?

Look at the endless miles that lie between us . . .

shadowy mountain ranges, seas that surge and thunder.

No, you, colossal, shameless – we all followed you,

to please you, to fight for you, to win your honour

back from the Trojans.

"This war is stupid and pointless. It's not our country and it's not our fight," is a view typical of those recorded by Guardian photographer and film-­makerSean Smith when he was embedded among US troops in Iraq.

The Odyssey is a poem as full of twists and turns as the mind of its wily hero, Odysseus. It contains flashbacks, embedded narratives, exotic locations, fairytale characters and a chronology – sometimes stretched, sometimes compressed – that covers a decade. The Iliad, in contrast, is a linear tale, circumscribed in geography and time-frame: we are placed variously in the Greeks' camp, the plain outside Troy, the city itself, and in the gods' home on Mount Olympus. Its characters are nearly all soldiers and gods, with mere bit parts for women, children and other non-combatants. It covers about 40 days during the 10th year of the war.

One of its most arresting characteristics, however, is the way it casts us forward and back, hinting at both a lost, peaceful world "back home", and the horrors of the post-conflict world to come. This is a quality that does much to lend the poem its pathos, and its constant sense of loss. Take its regularly used epithets: these familiar phrases ("wine-dark" sea, "rosy-fingered" dawn) have often been seen as simply as the more or less meaningless metrical building blocks that would have helped a bard to improvise lines of verse on the hoof. Sometimes, though, they seem to be carefully ­chosen. The last line of the epic is "And so they buried Hector, breaker of horses." That epithet, "breaker of horses", has been used of the hero ­dozens of times, yet it never ceases to stop me in my tracks. Breaking horses is a gentle art, the occupation of peacetime (even if those horses are being readied for future war). None of that for Hector now. There's a curious resonance between that line and an account, again published in Carey's collection, by a young farmhand who fought on the other side of the Dardanelles, in Gallipoli, in 1915. The lad is on sentry duty in the trenches. "I knew the next sentry up quite well. I remembered him in Suffolk singing to his horses as he ploughed. Now he fell back with a great scream and a look of surprise – dead."

Lost peacetime is, however, most often conjured up through the poet's imagery – in which we are often invited to imagine an act of great violence with the help of similes drawn from a pastoral world far from the battlefields of Troy. In the 11th book, the Greek warrior Ajax slowly withdraws from a bout of hand-to-hand fighting:

Like a stubborn ass some boys lead down a road . . .

stick after stick they've cracked across his back

but he's too much for them now, he rambles into a field

to ravage standing crops. They keep beating his ribs,

splintering sticks – their struggle child's play

till with one final shove they drive him off

but not before he's had his fill of feed.

In book 13, an arrow bounces off Menelaus's shield like chickpeas off a shovel; the following book has a boulder thrown by Ajax that sends Hector "whirling like a whipping top". Such humble, almost humorous images have a cumulative effect, creating a lightly sketched vision of a parallel world that sits at the back of the mind as we absorb the "foreground" action of the battle for Troy. Occasionally, such images contain their own violence, blurring into to the scenes they are helping us conjure. In the 12th book, the armies are said to fight like farmers rowing over a disputed a boundary stone – war writ small.

It is the Trojans, meanwhile, who provide the most obvious focus for the fragility of civilian life, and the horrors that await the city's old, its women, and its very young. One feature of the poem is that it accords equal dignity to both sides in the war: the Trojans are not dehumanised into "ragheads" or "gooks". In book six comes the ­famous, moving scene in which Hector, returning to the city after a bout of ­battle, encounters his wife Andromache and son Astyanax. This is a passage of tenderness and tearing grief, as we witness the hero's love for his wife and hers for him; and the sweet fragility of their child. It is this passage that helps Samet find in Hector the blueprint of the "citizen soldier", a warrior fighting to save his home and his values – a neat Americanisation.

Andromache appeals to her husband to use defensive tactics, to stop leading his men from the front. She is already a victim of war: her father and seven brothers have been killed in a previous conflict by Achilles himself; her mother is dead, too. "You, Hector – you are my father now, my noble mother, / a brother too, and you are my husband, young and warm and strong! / Pity me please," she begs. Hector ­sorrowfully refuses: honour dictates he must lead his men in the field, though he has ­little doubt of the defeat that is coming. It is not so much the pain of his parents, his brothers, dying that haunts him, he says.

"That is nothing, nothing beside your agony

when some brazen Argive hales you off in tears.

wrenching away your day of light and freedom!

Then far off in the land of Argos you must live.

labouring at a loom, at another woman's beck and call,

fetching water at some spring, Messeis or Hyperia,

resisting all the way –"

The child Astyanax recoils at the sight of his father's frightening plumed helmet. Hector picks him up, and Andromache smiles through her tears. He prays that the boy might one day be prince of the Trojans, their best fighter, better even than his father, "a joy to his mother's heart".

In antiquity, those encountering the poem would probably have been familiar with two other epics, now lost, that dealt with later parts of the ­Trojan war story (these are known as The Little ­Iliad and The Sack of Troy). The ­Odyssey fills in some blanks, not least the story of the wooden horse. Later come those Athenian fifth-century tragedies that develop stories begun in The Iliad: Aeschylus's Agamemnon, and Euripides's plays Hecuba and The Trojan Women, which deal with the calamitous fall-out of the war on its female victims – its "collateral damage".

From such texts we know how right, and how wrong, Hector is. We know that Andromache will, yes, be dragged into slavery. But we also know that his aspirations for his son are empty; even the infant's name is a cruel joke ­(Astyanax means "lord of the city"). The baby will be flung over Troy's ­ramparts by the victorious Greeks – a scene that appears in The Trojan Women.

It is perhaps in the relationships between the combatants that modern soldiers might most readily see their own emotions mirrored. In his book Achilles in Vietnam: Combat Trauma and the Undoing of Character, American psychiatrist Jonathan Shay finds parallels between the pathologies of ­Vietnam veterans whom he has treated, and Homer's Achilles. He argues that Achilles is suffering from what we would now call combat trauma, the death of Patroclus causing his character fatally to unravel. In particular, Shay compares the comradeship and passionate loyalty of American soldiers in Vietnam to that between Achilles and Patroclus – who grew up together, fought alongside each other, and whose relationship is the subject of some of Homer's most tender writing. In book 16 – shortly before he agrees to let Patroclus enter the fighting – Achilles finds him weeping:

"Why in tears, Patroclus?

Like a girl, a baby running after her mother,

begging to be picked up, and she tugs her skirts,

holding her back as she tries to hurry off – all tears

fawning up at her, till she takes her in her arms . . .

That's how you look, Patroclus, streaming live tears . . ."

Such fierce tenderness is echoed in the conversation of today's British troops fighting in Iraq and Afghanistan. Former Guardian war reporter Audrey Gillan was, in 2003, embedded with the Household Cavalry in Iraq. The regiment was initially reluctant to host a female journalist, but she was later told by the driver of the personnel carrier that became her home "Don't worry, I will never, ever leave you. I will pick you up and carry you if I have to."

In 2008, Gillan spoke to soldiers from the Princess of Wales's Royal Regiment who had been involved in a particularly brutal firefight in Basra four years earlier. Lance Corporal Martin Hill remembered the end of a fellow soldier: "He was dead. You could see his skin changing colour and his eyes were dilated. We went through every emotion possible then. Blokes were screaming out and crying." This is a long way from ramrod backs and stiff upper-lips.

When Antilochus brings Achilles the news of Patroclus's death in book 18,

"A black cloud of grief came shrouding over Achilles

Both hands clawing the ground for soot and filth,

he poured it over his head, fouled his handsome face

and black ashes settled on to his fresh clean war-shirt,

Overpowered in all his power, sprawled in the dust,

Achilles lay there, fallen . . .

tearing his hair, defiling it with his own hands . . ."

Shay records one of his patients recalling his own fury: "I really loved fucking killing, couldn't get enough. For every one of them I killed I felt better. Made some of the hurt went away [sic]. Every time you lost a friend it seemed like a part of you was gone. Get one of them to compensate what they had done to me. I got very hard, cold, merciless. I lost all my mercy."

Achilles also gets hard, cold, merciless. Even by the standards of The Iliad, his killing spree is grotesque. He cannot sleep or eat; he thinks only of killing: "what I really crave / is slaughter and blood and the choking groans of men". He slakes his bloodthirst by felling men, by filling the waters of the Scamander so full of bodies and gore that the river deity himself rises up from the depths in anger. It is "all day permanent red", to borrow the memorable title of one of ­Christopher Logue's ­poetic reimaginings of The Iliad.

Achilles captures 12 Trojan men whom he will sacrifice on Patroclus's pyre – again, even by the standards of The Iliad, a horrific act; today, we would call it a war crime. In book 21, he downs the Trojan prince Lycaon. You captured me once before, says Lycaon, but then, merciful, you spared my life. Do the same now. Achilles responds:

"Come, friend, you too must die. Why moan about it so?

Even Patroclus died, a far, far better man than you.

And look, you see how handsome and powerful I am?

The son of a great man, the mother who gave me life

a deathless goddess. But even for me, I tell you,

death and the strong force of fate are waiting.

There will come a dawn or a sunset or high noon

when a man will take my life in battle too –

flinging a spear perhaps

or whipping a deadly arrow off his bow."

After the loss of Patroclus, all life – ­Lycaon's, his own – is, for Achilles, utterly meaningless. We are all going to die; we (or at least you) may as well die now.

Yet this is an aberration: life does have meaning in The Iliad, a meaning that is bound up both with a warrior's kleos, the glory he achieves in the field, and, paradoxically, with a hero's willing, onward surge towards death. How are we, then, to read the poem amid the horrors and contradictions of our own wars, conflicts that have destroyed countless Andromaches and Astyanaxes? Bleak as The Iliad is, it is made all the bleaker by its divine characters. The poem's gods, who urge on the fighters and intervene to help their favoured heroes, are flimsy and flippant compared to their mortal counterparts, a source of troubling light relief rather than profundity. The life-and-death struggles of the human characters seem weightier and more agonisingly present when set against the meaningless existence of the gods. This is a hard world: the war isn't "for" anything, certainly not some greater good, but is merely part of the blind workings of an inexplicable fate that even Zeus, king of the gods, must bow to. When the warriors die, there are no flights of angels to sing them to their rest, only the prospect of a ghastly, ghostly, absence of meaning.

As Hector's soul departs his dying body, it does so "wailing his fate / leaving his manhood far behind, / his young and supple strength". The Iliad is a cavalcade of loss, an endless ­parade of men summoned briefly to life only to be consigned to death – such as young Gorgythion in the eighth book, subject of one of the poet's most poignant similes:

"As a garden poppy, burst into red

bloom, bends,

dropping its head to one side,

weighed down

by its full seeds and a sudden spring

shower,

so Gorgythion's head fell limp over

one shoulder,

weighed down by his helmet."

To post-first-world-war readers, it is hard not to add a further layer to these lines – Flanders fields a carpet of blood-red poppies.

At the end of the poem comes the scene between Priam and Achilles, when the frail, grieving father finds it in himself to kiss those "terrible, man-­killing hands / that had slaughtered Priam's many sons in battle", when ­Achilles sees reflected in the face of Priam the likeness of his own beloved father. Weil underestimated the power of this passage. Achilles is not simply an unfeeling "thing", reduced by the unspeakable power of force. The truth may be harder to take. He is at the same time a mass slaughterer and the gentlest of men. Only a few lines of verse stand between the Achilles who wipes away the tears of his beloved Patroclus and the one who piles up hecatombs of the Trojan dead. Find in this comfort, if you can.

• This article was amended on 2 February 2010. In the original, Robert Fagles was said to have recited the 1,000 lines of The Iliad in Greek during a West Point visit. This has been corrected.